Toilets gurgling... what's wrong here?

Two nights ago, the bathtub in the second story bathroom was used and fully drained all at once. I happened to be near the main floor bathroom and I heard the toilet gurgling/bubbling at some point during the tub draining. I could see large air bubbles coming out into the bowl. I tried flushing this toilet and the water wouldn't go down (raised up to near the top of the bowl). I grabbed a plunger and tried to plunge on it a few times and it did not go down. In the past, when this toilet flooded it would be extremely easy to plunge. I left the bathroom and came back about 5 min later.. the toilet was completely empty of water. Fast forward to today... as I took a shower (again, upstairs bathroom but not the same upstairs bathroom as the bathtub) I could hear the toilet gurgling/bubbling.

So far, I haven't seen any bad repercussions (water on the floor, ceiling, carpet, etc) but I am getting nervous something may be on the way soon... which is why I need your help. What could be the problem here? To me it seems like it is either a) an issue with the main sewer line or b) an issue with the venting in the house (possibly plugged?). I certainly could be wrong there.. and am hoping to get some suggestions on how I can diagnose between the two possibilities or potentially other possibilities that I hadn't thought of.

You have a partial obstruction in the main line. When you drain the tub, the line fills up and the water pressure and trapped air try to come out the easiest path, and that appears to be that toilet. It's partial, because eventually the water does drain past that blockage. It only tends to get worse. There may be a low spot (belly) in the pipe run that has accumulated crud, or it could be roots, or something else caught in the pipe, or worst case, the line may have collapsed or been crushed somehow. You can't flush a toilet when the pipe it drains into is already full!

yeah i had no idea who to call. Crap. At any rate, they showed up for two hours tonight. Did 4 runs.... two with a 'large' blade and 2 with a smaller blade. There was an obstruction near the turn at the city line... so he thinks. He could hear water sloshing around somewhere between 5 and 20' just outside the house (his starting point was at the access in my basement). Might be a belly as you say. Hopefully not major. Ugh. I'm running a camera tomorrow to better understand the issue but at this point I have no idea what it is and the partial block is getting worse.

OK so for an update: the camera crew was just here and they said 'sorry we can't see anything since the line is still full'. Now they're bringing another crew out to work the line again (snaking). Yay. So far I've only had to pay for the camera crew ($150)... they didn't charge for the first snaking so that I guess was nice. The guy said "It looks to be a major problem but who knows we'll see".

OK another update: so after the camera, they sent out a "Very experienced" plumber to try and re-snake the line. He worked it for 1.5hrs. He first used a 1.5" diameter blade and was able to get all the way to the city line, but it was 'tight' starting at 60' and "tighter" at about 80'. He then replaced the blade with a larger 3" diameter blade and it was easy going until about 80'. He worked that same spot at 80' for 15 min and was never able to move an inch.

Unfortunately, neither the 1.5" blade or the 3" blade were able to clear the issue enough to actually drain the pipe. So again, I was dealing with a pipe that most likely was still filled with sewage which would prevent camera inspection. Indeed, we went back in with a camera, just in case... but were still unable to see anything since it was still sewage-filled. Today, I'm receiving bids to do a short spot repair at the area where the camera couldn't get in. I'd have them add a cleanout access at that spot, then we'd have much more torque available to hopefully be able to get in and clean it enough to get a camera in. At that point, I'd have a better indication of what the problem is so I could get on with fixing it asap. Thoughts? By the way, the distance from where the new access point to the city tap would be about 25'.